Intrafusal fibers of muscle spindles are highly modified skeletal muscle cells essential for control of movement and posture. These multinucleated cells express a spindle-specific isoform of myosin heavy chain, alpha-cardiac MHC. The basic hypothesis for the proposed project is that motor neurons, sensory neurons, and thyroid hormone interact to induce and maintain expression of alpha-cardiac MHC in subsets of intrafusal fiber myonuclei. The principle investigator proposes to investigate the roles of these factors in regulating alpha-cardiac MHC gene expression in normal, denervated, hyperthyroid and hypothyroid rats. The experiments are designed to test: 1) whether subsets of intrafusal myonucei are induced to express alpha-cardiac MHC during spindle formation by innervation or elevated thyroid hormone; and 2) whether alpha-cardiac MHC is an isoform transiently expressed during the differentiation of extrafusal fibers which persists in intrafusal fibers, and whether innervation is the factor responsible for its persistence in intrafusal fibers.